The invention relates to a valve device for an aeration and air evacuation pipe of an aeration and air evacuation device arranged in an upper tank part with the pipe leading away from a motor fuel tank. The valve device has a housing which exhibits a connecting duct and an aeration and air evacuation duct, in which a negative pressure valve which opens in the case of negative pressure in the tank and a positive pressure valve which opens in the case of positive pressure in the tank are arranged between a connecting duct and the aeration and air evacuation duct, the positive pressure valve exhibiting a valve element guided raiseably and resting on a valve seat by the effect of gravity.
In the case of aeration and air evacuation devices installed in motor fuel tanks, it is necessary for the refuelling height to be limited to a definite dimension so that the aeration and air evacuation device still remains above the maximum fuel level in the fully refuelled state.
This is achieved by means of a valve device located in the aeration and air evacuation pipe outside the fuel tank, which establishes a connection of the tank interior to the atmosphere only when a positive pressure which builds up successively in the motor fuel tank during refuelling has attained a value which is greater than the pressure which results from the static height of the column of fuel required in the refuelling pipe connection to actuate the automatic cut-out device of a discharge gun. It is only in this way that the required volume of air is maintained in the fuel tank for ending the refuelling operation.
In the customary aeration and air evacuation devices an escape of motor fuel is possible through their aeration and air evacuation pipe from a specific sloping position and particularly in the inverted position of the fuel tank. For this reason the valve device of such aeration and air evacuation devices is equipped both with a positive pressure valve and a negative pressure valve and also with a safety valve. The latter exhibits a weight component which moves automatically into its closed position by the effect of gravity above a specific sloping position of the valve device. The valve element of the positive pressure valve is braced here against a compression spring which generates its closing pressure in customary manner, as the valve device according to German Utility Model 7,419,074 shows for example.
However, an aeration and air evacuation device which has already been proposed according to commonly assigned pending U.S. patent application, Ser. No. 33288, filed June 13, 1988 based, German Application P 3,719,834.3-13, is conceived so that it also assumes the function of a safety valve in a sloping or inverted position of the motor fuel tank. The valve device to be provided in its aeration and air evacuation pipe accordingly only requires to be equipped further with a positive pressure valve and a negative pressure valve. The required positive pressure value must be kept very small in this case (approximately 80 mm WG), which could only be achieved by bracing the valve element against a compression spring if a compression spring with a suitably low spring constant and a large cross-section of the valve seat were provided, which however would necessitate a great spring length. In this case it would be necessary for safety and functional considerations to ensure that no positive pressure, or none higher than that necessary to limit the level during refuelling, occurs in road service.
The invention starts here. An underlying object of the invention is to provide a valve device, the positive pressure valve of which requires no compression spring and ensures the control of a minimum pressure value with the smallest pressure deviations. It is also intended to be characterized by small dimensions, simple construction and reliable operation of its positive pressure valve, even at low temperatures.
Starting from a positive pressure valve of a valve device according to German Utility Model 7,419,074, this object is achieved according to the invention by providing an arrangement wherein a housing which exhibits a connecting duct and an aeration and air evacuation duct, in which a negative pressure valve which opens in the case of negative pressure in the tank and a positive pressure valve which opens in the case of positive pressure in the tank are arranged between a connecting duct and the aeration and air evacuation duct, the positive pressure valve exhibiting a valve element guided raiseably and resting on a valve seat by the effect of gravity.
The positive pressure of valve of the valve device according to the invention therefore operates with functional parameters which can be executed to very close tolerances in production. These functional parameters are the pressure-stressed surface, the size of which is also determined by sloping position of the valve element in its closed position, and the weight of the valve element, it being possible to determine any desired closing force to close tolerances by a corresponding coordination of the characteristic surface and weight.
The arrangement of the valve seat on a downward sloping duct end face ensures that it is impossible for condensate to accumulate within the duct, which might jam the valve element by freezing fast at low temperatures.
Since no further valve components are required apart from valve element and duct, the positive pressure valve can be designed advantageously in terms of space economy and production technology, and it is particularly suitable for aeration and air evacuation devices according to the above noted pending patent application corresponding to German application p 3,719,834.3-13.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.